In the recent drive for higher integration and operating speeds in LSI devices, the pattern rule is made drastically finer. The photolithography which is currently on widespread use in the art is approaching the essential limit of resolution determined by the wavelength of a light source.
As the light source used in the lithography for resist pattern formation, g-line (436 nm) or i-line (365 nm) from a mercury lamp was widely used. One means believed effective for further reducing the feature size is to reduce the wavelength of exposure light. For the mass production process of 64 MB dynamic random access memories (DRAM, processing feature size 0.25 μm or less) and later ones, the exposure light source of i-line (365 nm) was replaced by a KrF excimer laser having a shorter wavelength of 248 nm.
However, for the fabrication of DRAM with a degree of integration of 256 MB and 1 GB or more requiring a finer patterning technology (processing feature size 0.2 μm or less), a shorter wavelength light source is required. Over a decade, photolithography using ArF excimer laser light (193 nm) has been under active investigation.
It was expected at the initial that the ArF lithography would be applied to the fabrication of 180-nm node devices. However, the KrF excimer lithography survived to the mass-scale fabrication of 130-nm node devices. So, the full application of ArF lithography started from the 90-nm node. The ArF lithography combined with a lens having an increased numerical aperture (NA) of 0.9 is considered to comply with 65-nm node devices.
For the next 45-nm node devices which required an advancement to reduce the wavelength of exposure light, the F2 lithography of 157 nm wavelength became a candidate. However, for the reasons that the projection lens uses a large amount of expensive CaF2 single crystal, the scanner thus becomes expensive, hard pellicles are introduced due to the extremely low durability of soft pellicles, the optical system must be accordingly altered, and the etch resistance of resist is low; the postponement of F2 lithography and the early introduction of ArF immersion lithography were advocated (see Proc. SPIE Vol. 4690 xxix).
In the ArF immersion lithography, the space between the projection lens and the wafer is filled with water. Since water has a refractive index of 1.44 at 193 nm, pattern formation is possible even using a lens with NA of 1.0 or greater. Theoretically, it is possible to increase the NA to 1.35. The resolution is improved by an increment of NA. A combination of a lens having NA of at least 1.2 with ultra-high resolution technology suggests a way to the 45-nm node (see Proc. SPIE Vol. 5040, p 724).
However, for resist materials, as the circuit line width is reduced, the influence of acid diffusion on contrast becomes more serious. This is because the pattern size is approaching the diffusion length of acid, and leads to a lowering of mask fidelity and a degradation of pattern rectangularity. Therefore, in order to take full advantage of the reduced wavelength of a light source and the increased NA, it is necessary to increase the dissolution contrast over prior art materials and to restrain the acid diffusion.
With respect to the immersion lithography, several problems associated with the presence of water on resist were pointed out. Because the photoacid generator in the resist film, the acid generated therefrom upon exposure, and the amine compound added to the resist as a quencher can be leached in water in contact with the resist film, pattern profile changes occur. The pattern collapses due to swelling of the resist film with water.
With respect to the leaching of resist components into water, a study started from the standpoint of preventing the projection lens of the lithography system from contamination. Several lithography system manufacturers proposed the limit of leach-outs.
For overcoming these problems, it was proposed to provide a protective coating of perfluoroalkyl compound between the resist film and water (see the 2nd Immersion Workshop, Jul. 11, 2003, Resist and Cover Material Investigation, for Immersion Lithography). The provision of such a protective coating avoids direct contact between the resist film and water and inhibits the resist film from being leached with water.
However, protective coatings made of perfluoroalkyl compounds use fluorocarbons like Freon® as the diluent for controlling a coating thickness. As is well known, the use of fluorocarbons is a consideration in view of environmental protection. In addition, the protective coating must be stripped using fluorocarbon, prior to development of the resist film. Therefore, special units for coating and stripping of the protective film must be added to the existing system. Fluorocarbon solvents add to the expense. These factors raise serious problems on practical use.
One means proposed for mitigating practical drawbacks of the protective film of solvent stripping type is a protective film of the type which is soluble in alkaline developer (JP-A 2005-264131). The alkali-soluble protective film is epoch-making in that it eliminates a need for a stripping step or a special stripping unit because it can be stripped off at the same time as the development of a photoresist film.
The ArF immersion lithography systems commercially available at the present are designed such that water is partly held between the projection lens and the wafer rather than immersing the resist-coated substrate fully in water, and exposure is carried out by scanning the wafer-holding stage at a speed of 300 to 550 mm/sec. Because of such high-speed scanning, water cannot be held between the projection lens and the wafer, and water droplets are left on the surface of the resist film or protective film after scanning. It is believed that residual droplets cause defective pattern formation.
To eliminate the droplets remaining on the surface of the photoresist or protective film after scanning, it is necessary to improve the flow or mobility of water on the relevant coating film. It is reported that the number of defects associated with the immersion lithography can be reduced by increasing the receding contact angle of the photoresist or protective film with water. See 2nd International Symposium on Immersion Lithography, 12-15 September, 2005, Defectivity data taken with a full-field immersion exposure tool, Nakano et al. It is noted that the method of measuring a receding contact angle includes a sliding method of inclining a substrate and a suction method of sucking up water, with the sliding method being widely accepted.
If residues are left on the resist film after development, there arise defects which are known as blobs. The blob defects occur because the protective coating or resist material is precipitated during rinsing after development and deposited on the resist film again, and are found more often when the resist film after development is more hydrophobic. For the resist for use in the immersion lithography in combination with a protective coating, if mixing occurs between the protective coating and the resist coating, the hydrophobic protective coating can be left on the surface of the resist coating after development, leading to blob defects on the resist coating. It is then necessary to prevent intermixing between the protective coating and the resist coating so that no protective coating is left after development.
In the electron beam writing lithography, there arises a problem that electric charges accumulate in the resist coating during exposure. The charge accumulation may be avoided by forming a water-soluble antistatic coating on the photoresist. The antistatic coating is formed of materials comprising a water-soluble polymer and an ammonium salt of sulfonic acid added thereto. The negative charges generated by electron beam exposure are transferred through the antistatic coating to the earth side of a wafer chuck where they are released. However, the antistatic coating, when formed on the photoresist coating, adds an additional coating step and a coating material cost to the overall process, which are undesirable. It would be desirable to have a photoresist having an antistatic function, eliminating a need for a separate antistatic coating.